Sloppy casting call announcements have revealed key plot points and characters before, so it's easy to believe that may have just happened again with the two-part The Avengers: Infinity War.
As Screen Rant points out, an alleged casting call announcement for the sequels spells it out.
Does that mean key Guardians of the Galaxy comics character Adam Warlock and his villainous alter ego Magus are going to show up? Sure seems like it, but obviously its only a rumor at this point.
That's not the only thing revealed by this alleged casting announcement. It goes on to list the main actors and their characters in the film. That list includes Tom Hiddleston as Loki, who has also been rumored for a while, but this could (almost) confirm it.